Extrusion processes have long been used to coat numerous items such as pipes, cables and wires with various plastics. Conventional extrusion processes use a chamber to hold the material used for extrusion. Mechanical pressure is applied via a compression ram at one end of the chamber, and the material is forced through a patterned opening at the other end to create an extruded form or a coating over an item. For example, a copper wire can be coated with a process that extrudes a polymer from an extrusion die, drawing the polymer onto the wire in a vacuum environment, and applying the polymer to the wire inside the annular extrudate or melt cone.
Extrusion methods have been used to create coatings, sheaths or tubing for protecting or deploying various medical devices that use wires. One example of an extruded polymeric sheath for coating wires is described in “Co-Extruded, Multi-Lumen Medical Lead,” Borgersen et al., U.S. Patent Publication 2002/0183824 issued Dec. 5, 2002. The electrically insulating sheath can be used with, for example, implantable cardiac leads for delivering pacing pulses and defibrillation shocks, or sensing a cardiac electrogram (EGM). The body sheath is co-extruded in a co-extrusion process with materials of differing durometers in differing axial sections thereof to create a unitary body sheath. Another method for extruding material onto a wire is described by Solar and others in “Method of Manufacturing a Guidewire with an Extrusion Jacket”, U.S. Patent Publication 2002/0084012 issued Jul. 4, 2002. A corewire is fed into an extrusion device and a material is extruded onto the corewire while a gripping apparatus pulls the corewire through the extrusion device to create a coating or extrusion jacket.
Extruded polymeric tubing has been used for medical grafts. An example of a medical graft that has an additional exterior support structure is disclosed in “Endoluminal Graft with Integral Structural Support and Method for Making Same”, Edwin et al., U.S. Pat. No. 6,053,943 issued Apr. 25, 2000. The structurally supported tubular graft may include a spiraling beading element that is co-extruded with the support structure. The spiraling support structure, which is on the outside of the graft, allows for the expansion of the graft. Unconnected ends of the support structure may have outwardly protruding barbs that upon expansion of the graft secure the graft within a blood vessel or body lumen. The support structure is designed to constrain the tubing of the graft.
An extruded sheath assembled with an expandable stent may be used to constrain the stent until it has reached its areas of deployment. One such sheath is described in “Methods of Forming a Coating for a Prosthesis”, Harish et al., U.S. Patent Publication 2002/0122877 issued Sep. 5, 2002. The sheath can be used to constrain an expandable stent until it has reached its areas of deployment. The method for manufacturing the associated stent assembly includes steps of forming a sheath, placing the sheath upon an implantable device or endoluminal prosthesis, and heating the sheath to coat the device or prosthesis. The coating created from the sheath may be used for the delivery of an active ingredient and may have a selected pattern of interstices for allowing a fluid to seep through the coating in the direction of the pattern created.
There are a number of dipping and spraying methods that have been used to apply coatings to the stent framework. A less common method uses injection molding, one example being described in “Polymer-Coated Stent Structure”, Loeffler, U.S. Pat. No. 5,897,911 issued Apr. 27, 1999. An exterior mold around the stent controls the thickness of polymer on the exterior surface of the stent. Alternatively, this method may use a preformed sheath of polymer fitted to the interior of the stent whereby a subsequent application of a polymer coats the exterior of the stent.
Although extrusion methods have been developed to extrude coatings on medical devices such as electrical leads and guidewires, little research has focused on the extrusion of a polymeric or drug-polymer coating onto a stent framework. Recent clinical studies on drug-coated vascular stents indicate much therapeutic benefit with the addition of stent coatings that contain pharmaceutical drugs. These drugs may be released from the coating while in the body, delivering their patent effects at the site where they are most needed. Thus, the localized levels of the medications can be elevated, and therefore potentially more effective than orally- or intravenously-delivered drugs that distribute throughout the body.
It would be desirable to have a process for coating and covering a stent with a wide variety of polymers, drugs, and other types of coating materials. The desired process may not require heating and would therefore have minimal impact on pharmaceutical drugs and compounds incorporated into the stent coating. The desired process would use little, if any, solvent, and reduce the amount of drug wasted during typical dipping and spraying cycles. The process would require fewer undesirable chemicals, and reduce or eliminate drying time needed for evaporation of a solvent. The process would allow large quantities of drugs to be included in the coating, and allow various forms of drugs such as micronized powdered drugs and encapsulated drug microspheres to be included within the coating. The method would provide a well-controlled coating thickness, allow a large percentage of drugs within the coating, require little time for application of the desired coating, and overcome the deficiencies and limitations of other coating methods described above.